Mechanisms of charge transfer at the chemically derivatized interface: The Ni/[Ni^II(CN)Fe^II/III(CN)₅]^2-/1- system as an electrocatalyst

The [Ni(NC)Fe(CN)₅]^2-/1- derivatized nickel electrode represents an electrocatalytic surface for a variety of oxidations and reductions. This surface exhibits a unique dependence of redox potential on supporting electrolyte cation, which allows for a direct analysis of the effect of surface redox potential on the electrocatalytic rate constant. Two electrocatalytic systems have been evaluated with respect to surface redox potential: the one-electron reduction of Fe³⁺(aq) and the two-electron (two-proton) oxidation of ascorbic acid. Mediated charge transfer is found to be an operational electron-transfer mechanism in both cases, with the bimolecular surface species to solution species charge transfer being rate limiting. In the case of Fe³⁺(aq) reduction Marcus theory is found to yield a good description of the relationship between surface redox potential and the electrocatalytic rate constant. Pseudo-first-order rate constants as large as 0.15 cm/s (in LiNO₃ supporting electrolyte) have been observed for this reaction. The ascorbic acid oxidation rate constant is found to be ∼10^-3 cm/s. This rate constant is independent of surface redox potential, suggesting that the transfer of the second electron is rate limiting.